1. Field of the Invention
The present invention relates to the field of semiconductor testing equipment and, more specifically, to the field of probe cards for semiconductor test systems.
2. Description of the Related Art
In the manufacture of semiconductor devices, it is advisable that such components be tested at the wafer level to evaluate their functionality. The process in which die on the wafer are tested is commonly referred to as "wafer sort". Testing and determining design flaws at the die level offers several advantages. First, it allows designers to evaluate the functionality of new devices during development. Increasing packaging costs also make wafer sorting a viable cost saver, in that the reliability of each die on the wafer may be tested before incurring the high costs of packaging.
Wafer sorting typically involves the use of probing technology wherein a probe card containing probing features engages the die so as to connect the bond pads of the die to a tester. With recent advances in silicon technology, microprocessor performance is becoming limited by chip to package interconnections. Three primary processes--wirebonding (WB), tape automated bonding (TAB), and controlled collapse chips connection (C4)--are used to interconnect a chip to a package. The C4 technology utilizes solder bumps, comprised of mostly lead and some tin, to interconnect the chip to a package. During wafer sorting the probing features of the probe card contact the solder bumps.
Most categories of probing utilize some form of scrub to ensure electrical contact between the probing feature and a bond pad or solder bump. FIG. 1 shows a solder bump 10 with an oxide layer 12, typically a non-conductive film formed on the surface of exposed lead and tin. Generally, scrub applies to any non-conductive layer that produces a barrier between the probing features of a probe card and the base metal of a bond pad. The purpose of the scrub is to break through the non-conductive layer in order to establish good electrical contact between the probing features and the base metal of the bond pads or solder bumps. Scrub occurs when the handler forces the wafer, and, subsequently, the bond pads of a die, against the probe features on the probe card causing the probe feature to deflect. The scrub is generated by a small horizontal movement of each probe feature across the surface of each corresponding bond pad as the probing features deflect. As the probing features move across the bond pads or solder bumps, they penetrate the non-conductive oxide layer thereby establishing good electrical contact between the probing features and the bond pads or solder bumps.
In the process of making contact with a bonding pad or a solder bump, a deposit accumulates on the probing feature of the probe card. For example, when a solder bump is comprised of primarily lead and some tin, a deposit consisting primarily of lead, lead oxides, and lead alloys adheres to the probing feature. FIGS. 2A-2C roughly demonstrate the wafer sort process. FIG. 2A illustrates a portion of probe card 20 with microspring probing features 22 coupled to multi-layer ceramic space transformer 21 and probing feature tips 24. During wafer sort, as roughly shown in FIG. 2B, microspring tips 24 touch down on solder bumps 26 which are part of integrated circuit device 28. The lead tin alloy is extremely malleable and easily adheres to the metal or metal alloy probing feature. FIG. 2C depicts microsprings 22 following a wafer sort during which solder deposits 29 have accumulated on microsprings 22 bridging the small gap between microsprings. The probing features of probe cards are spaced very close together. For example, in one configuration, a microspring probe card may have 1,500 microsprings (40 mils tall; 5.5-6 mils diameter) with a minimum pitch (spacing) between the microsprings of 225 micron.
FIGS. 3, 4 and 5 depict additional types of probing features although it is appreciated that other types of probing features are available. FIG. 3 illustrates a portion of a probe card 30 with probing features 34 contained within a tubular housing. The probing feature or "beam" deflects upon contact with a solder bump by buckling or bending. The probing features are coupled to a multi-layer ceramic space transformer 31. A third example of a probing feature is shown in FIG. 4, which illustrates a "cantilever needle" probe card 40. The cantilever needles 42 are connected to a printed circuit board 41 and held in place by an epoxy ring 43. Finally, FIG. 5 depicts a cross sectional view of a "membrane" probe card 45 with probing features 48 connected to a flexible printed circuit 46. As discussed above, the lead tin composition on the surface of the solder bump easily adheres to the metal or metal alloy probing feature without regard to the shape or type of probing feature utilized during the wafer sort process.
Stray particle and solder buildup contributes to high contact resistance between the probing feature's tip and the solder bump. High contact resistance causes inaccurate voltage levels during device testing due to the non-conductive layer produced across the probe tip. This may cause a device to incorrectly fail, resulting in unnecessarily lower test yields. Moreover, the accumulated buildup of solder deposits may bridge the small gap between probing features resulting in shorts or leakage currents, again, leading to unnecessarily lower test yields. Additionally, with respect to the buckling beam probing feature, illustrated in FIG. 3, the solder buildup may cause the probing feature to be lodged within the lower die plate. In order to ensure accurate wafer sort test results, deposits that adhere to a probing feature during wafer sort must be removed.
An existing method for removing these deposits from the probing features is the abrasive cleaning method illustrated in FIG. 6 which depicts a portion of probe card 50 with probing features 54 extending from multi-layer ceramic space transformer 51. After deposits 54 build up on microsprings 52 during wafer sort, tips 56 of microsprings 52 are brought into contact with an abrasive paper 58. By moving probe card 50 in relation to abrasive paper 58, deposits 54 are removed from tips 56, however deposits 54 remain along the length of microspring 52, due to the adhesive nature of the solder. Typically probing features are abrasively cleaned once every 100 die.
The abrasive cleaning method is undesirable because the method (i) results in excessive erosion of the probing feature which dramatically reduces the lifetime of the probe card; (ii) only removes deposits from the tips of the probing features allowing the deposits to accumulate on the non-tip portion of the probing feature; and (iii) is less effective for removing solder from dirty probing features which have remained on a prober for an extended period of time where the solder has oxidized. Because the abrasive paper is flat and relatively non-compliant, the abrasive cleaning method is not suitable for removing deposits from the non-tip portion of the probing feature.
Another method used to remove deposits from a probing feature involves placing the probing feature in an acid-based composition. Tungsten cantilever needle probing features are typically placed in an acidic solution which dissolves the probing feature thereby removing the aluminum or lead-tin oxide buildup on the probing feature. The problem with this method is that the lifetime of the probe card is dramatically reduced since the tip of the probing feature is partially dissolved as it is exposed to the acid-based cleaning composition.
Thus, what is needed is a non-destructive method of removing unwanted deposits from probing features of a probe card which solves the problems associated with current removal techniques.